In exhaust systems of internal combustion engines, various catalytic converters containing heavy metals or noble metals as a catalyst have been used as an apparatus for cleaning exhaust gases in order to treat and remove harmful components in the exhaust gases, such as carbon monoxide or various hydrocarbons. These catalytic converters are classified into the following two types according to the configuration of catalysts used therein:
(1) Catalytic converter comprising a pellet-like catalyst prepared by supporting a metal catalyst on a granular carrier made of ceramics or the like, and a metal casing for accommodating the pellet-like catalyst; and PA1 (2) Catalyst converter comprising a so-called integrated catalyst prepared by supporting a metal catalyst on a tubular monolith carrier (hereinafter referred to merely as "monolith") inside of which a plurality of flow paths for passing exhaust gases are provided, and a metal casing accommodating the integrated catalyst and connected with exhaust pipes.
The catalytic converters (2) have been more predominately utilized as compared with those of the catalytic converters (1) because the catalytic converters (2) do not show abrasion due to collision between pellets as observed in the catalytic converters (1) and have relatively miniaturized the apparatus. In the catalytic converters (2), in order to securely mount a monolith member within a metal casing, a monolith-holding element is wound around a monolith.
In many cases, the monoliths made of ceramics or metals have been used in order to impart a heat resistance thereto. Further, in order to reduce a flow resistance during passing of exhaust gases and enhance an efficiency of the catalyst, there have been predominately utilized monoliths having a honeycomb structure, whereby more larger surface area can be assured in flow paths of exhaust gases.
As an appropriate structure of the metal casings, there has been adopted a two-piece clamshell structure in which the monolith member is sandwiched between shell halves of the metal casing and the mating portions of the shell halves are welded together, or a stuffing box structure in which the monolith member is inserted into the metal casing.
In either the clamshell or stuffing box structure, it is required that the thickness of the monolith-holding element is identical with or slightly larger than a clearance between an outer peripheral surface of the monolith and an inner peripheral surface of the metal casing in order to securely fix the monolith within the metal casing. This is because the decrease in retaining force of the monolith-holding element against the monolith causes inconveniences such as separation or displacement of the monolith within the metal casing, leakage of exhaust gases from the outer peripheral surface of the monolith, or the like, during operation. Occurrence of such inconveniences is highly detrimental to the catalytic converters.
Especially, in the case where a ceramic monolith is used in the catalytic converter, the clearance between the outer peripheral surface of the monolith and the inner peripheral surface of the metal casing becomes large during operation because the monolith itself exhibits a small thermal expansion while the metal casing exhibits a large thermal expansion. As a result, there has been a tendency that the retaining force of the monolith-holding element becomes decreased. In addition, in the production of the catalytic converter, in the case where such a monolith having an extremely size and showing a relatively large dispersion in its outer dimension is used, a retaining force required for securely holding the monolith cannot be often obtained upon assembling.
Accordingly, in order to obtain a desired retaining force of the monolith-holding element over a wide temperature range from a normal temperature to a high temperature during operation, it is required that the monolith-holding element can follow the difference in size of the clearance caused by fluctuated outer dimensions of the individual monoliths, or the change in the clearance due to difference in thermal expansion between the monolith and the metal casing.
In recent years, intense studies have been made on a method for fixing the monolith within the metal casing. As a result, as monolith-holding elements adapted to be fitted into a clearance between the outer peripheral surface of the monolith and the inner peripheral surface of the metal casing, there have been proposed monolith-holding elements prepared by forming an inorganic fiber material into various configurations. The techniques previously proposed for monolith-holding elements and problems caused thereby are described below.
Japanese Patent Application Laid-open (Kokai) No. 1-240715 (1989) discloses an elastic mat (monolith-holding element) composed of non-shot ceramic fiber which is sewed up and compressed in the thickness direction. However, such an elastic mat is compressed so as to reduce its thickness only when sewed up at a small sewing pitch. In the case where the small sewing pitch is used, there arise problems that the elastic mat is damaged, thereby resulting in lack of elasticity thereof, or that it becomes difficult to conduct delicate processing such as formation of labyrinth structures at opposite joining ends of the elastic mat. Further, there arise problems concerning the fraying, cutting or the like of threads at the sewed end portions of the elastic mat.
British Patent No. 2171180A discloses a mat-like product (monolith-holding element) prepared by vacuum-packing an inorganic fiber in a plastic bag. However, the plastic bag used in such a mat-like product does not have a flexibility. Further, since it is difficult to wind the mat-like product around the monolith in a close contact manner and provide labyrinth structures at opposite joining ends of the plastic bag, there arises a problem concerning sealing properties. Furthermore, there is such a inconvenience that the plastic bag film is likely to be broken upon transportation or assembling.
U.S. Pat. No. 4,693,338 discloses a monolith-holding element which comprises, in combination, a blanket prepared by highly compressing a ceramic fiber together with a small amount of a binder, and knitting yarns made of a !4j ceramic fiber. However, such a monolith-holding element requires complicated mounting procedures and assembling steps, and therefore, is unsatisfactory.
Japanese Patent Application Laid-open (Kokai) Publication No. 53-2753 (1978) discloses a heat-insulating element (monolith-holding element) prepared by compression-molding a fiber-based heat-insulating material such as ceramic fiber, silica fiber, glass fiber or the like together with an organic binder at a compression ratio higher than that in actual use. However, such a monolith-holding element is deteriorated in elasticity and shows accelerated thermal degradation such as softening or shrinkage in a high-temperature range, though it exhibits a thickness-restoring property in association with thermal decomposition of the organic binder. In addition, since the thickness of the monolith-holding element is thinner than the clearance, the monolith is caused to move within the casing before the thermal decomposition of the organic binder, thereby resulting in damage to the monolith.
Japanese Patent Application Laid-open (Kokai) No. 7-77036 (1995) discloses a catalytic converter having a heat-resistant and non-thermally-expansive ceramic fiber, which comprises a metal casing, a ceramic honeycomb catalyst (monolith) accommodated within the metal casing, and a ceramic fiber mat as a monolith-holding element fitted in an compressed state between an outer surface of the honeycomb catalyst and an inner surface of the metal casing. The catalytic converter disclosed therein includes the ceramic fiber mat having such a compression property which is not largely increased or decreased within a practically used temperature range. However, since no binder is contained in the afore-mentioned ceramic fiber mat, it is required to highly compress the ceramic fiber mat upon assembling. This causes deterioration in working properties and other problems such as breaking of the fiber due to lack of a mechanical strength thereof, scattering of the fiber or the like.
Japanese Patent Application Laid-open (Kokai) No. 57-56615 (1982) discloses an apparatus for cleaning exhaust gases, which comprises a metal casing, a ceramic monolith and a sealing element as a monolith-holding element fitted into a clearance between an inner circumferential surface of the metal casing and an outer circumferential surface of the monolith. The sealing element disclosed therein is composed of a ceramic fiber having a fiber diameter of 6 to 30 .mu.m, and has a bulk density of 0.1 to 0.35 g/cm.sup.3. Further, the sealing element is compressed such that the ratio between thicknesses before and after installation thereof is within the range of 2.7 to 8.7. However, since the sealing element has such a small bulk density and such a large compression ratio, there arises a problem concerning working properties upon assembling.
On the other hand, in the consideration of a heat resistance and cushioning properties of the ceramic fiber, as the monolith-holding element there have been proposed sheet elements prepared by blending the ceramic fiber with a thermally-expansive material such as vermiculite or the like for compensating a retaining force thereof.
Japanese Patent Application Laid-open (Kokai) No. 7-127443 (1995) discloses a ceramic honeycomb catalytic converter comprising a metal casing, a ceramic honeycomb catalyst (monolith), a gripping element (monolith-holding element) composed of a ceramic fiber mat disposed in an compressed state so as to retain the ceramic honeycomb catalyst by its restoring force, and a fixing member for securing the gripping element at the flowing direction of exhaust gases, the fixing member being possessed in the metal casing. However, the gripping elements made of a ceramic fiber mat is maintained in a non-compressed condition and therefore, must be highly compressed upon mounting, whereby there arise problems such as the deterioration of working properties and the breaking of the ceramic fiber itself. Further, there is a tendency that the honeycomb catalyst is damaged by a tightening force of the fixing member which is adapted to prevent a floating movement of the honeycomb catalyst.
In addition, Japanese Patent Publication (Kokoku) No. 58-17335 (1983) discloses a process for producing an integrated catalyst component (catalytic converter) comprising an integrated catalyst (monolith), a ceramic fiber molded-element which is separated in a circumferential direction of the integrated catalyst and wound around an outer circumferential surface of the integrated catalyst, and a casing securely receiving the integrated catalyst therein, which process comprises the steps of tightening the ceramic fiber molded-elements by means of compression rings having a separated structure from its outer peripheral side, covering end portions of the compression rings together with end faces of the ceramic fiber molded-elements by retaining rings, and accommodating the integrated catalyst within the casing. In such a catalytic converter produced according to the afore-mentioned technique, the ceramic fiber molded-elements are prevented from being damaged upon assembling and the end portions thereof is also protected from exhaust gases. However, since the clearance between the integrated catalyst and the casing, i.e., the clearance between the outer peripheral surface of the monolith and the inner peripheral surface of the metal casing is considerably fluctuated depending upon the difference in coefficient of thermal expansion therebetween, it cannot be expected that the ceramic fiber molded-elements exhibits sufficient supporting effects.
Meanwhile, ceramic monoliths composed mainly of cordierite have been most widely utilized because they are relatively less expensive and excellent in thermal dimensional stability. In accordance with the strengthening of regulations of exhaust gases, efforts have been made to enhance an efficiency of the catalyst by increasing temperature characteristics thereof, and to increase a catalytic surface area and decrease a heat capacity by reducing a wall thickness of the honeycomb catalyst. However, the reduction in wall thickness of the ceramic monolith is limited by the production conditions or the mechanical strength of the structure. In consequence, it has been recognized that metal monoliths are preferable as compared with the ceramic monoliths.
In general, the metal monoliths are composed of aluminum-containing ferrite-based stainless steel foils having a thickness of 50 to 100 .mu.m. Since among of the stainless steel foils other than the ferrite-based foils, for example austenite-based stainless steel foils show a large thermal expansion or martensite-based stainless steel foils are difficult to undergo bending, these are not put into practical use.
The ferrite-based stainless steel foils monoliths are prepared by winding a laminate composed of a flat foil and a corrugated foil into a form of roll and then bonding the opposite ends of the roll together by brazing to form a cylindrical material. Such metal monoliths have a reduced wall thickness which is 1/2 to 1/3 or less of the wall thickness of the ceramic monoliths. Therefore, nevertheless the metal monolith has the same size as that of the ceramic monolith, the metal monolith can show considerably larger opening rate and surface area, and considerably smaller heat capacity as compared with those of the ceramic monolith, resulting in achieving a high cleaning efficiency for exhaust gases. Further, the metal monoliths are excellent in dimensional accuracy upon the production and shows advantages such as a resistance to rupture or fracture. As a result, the metal monoliths are considered to be stably predominately utilized in future.
In catalytic converters using the afore-mentioned metal monoliths, a structure capable of integrally bonding the monolith to the surrounding metal casing is adopted in order to ensure bonding of the metal monolith by brazing. However, the afore-mentioned structure using the metal monolith can show various above-mentioned advantages required for a catalyst carrier as compared with the structure using the ceramic monoliths. Since a heat generated in the monolith is directly transferred to the surrounding metal casing, there arises a problem that the metal monolith is rapidly cooled during the idling operation.
As techniques concerning catalytic converters using metal monoliths, Japanese Utility Model Application Laid-open (Kokai) No. 1-80620 discloses a carrier for a catalyst for cleaning exhaust gases, which comprises a sheathed pipe, a metal monolith and a thermally-expansive sheet material interposed between the sheathed pipe and the metal monolith. However, when the metal monolith is exposed to exhaust gases having a temperature as high as not less than 800.degree. C., the heat resistance thereof is extremely deteriorated. In the case of the metal monolith disclosed in the above-mentioned Japanese Utility Model KOKAI, there arises such a problem that the honeycomb monolith is readily buckled by a pressure generated due to the thermal expansion of the thermally-expansive sheet material wound around an outer peripheral surface of the monolith.
In addition, in order to solve the afore-mentioned problems, Japanese Patent Application Laid-open (Kokai) No. 6-126191 discloses a catalytic converter using a metal monolith, in which the metal monolith is fixed in the catalytic converter in the same manner as used for fixing the ceramic monolith by the monolith-holding element. The catalytic converter disclosed in the Japanese Patent KOKAI comprises a monolith having a honeycomb structure prepared by winding a flat plate and a corrugated plate both made of metal foil into a form of roll, a metal ring having a short axial length and bonded to an outer peripheral surface of the monolith, a casing accommodating the monolith and a thermally-expansive sealing element fitted into a clearance between the ring and the casing.
In the catalytic converter disclosed in the Japanese Patent KOKAI, since the thermal expansion of the monolith becomes larger than that of the metal ring or the like as a monolith-holding element with temperature rise, a high tightening force is temporarily exerted on the monolith by the metal ring or the like as a monolith-holding element. Besides, since the thermally-expansive sealing element made of a ceramic wool further compresses the monolith by means of the metal ring, the monolith is likely to be buckled. As a result, there arises such a problem that the monolith is apt to cause a floating movement within the metal casing during operation.
Furthermore, WO 94/24425 discloses a mat comprising an integrated composite sheet composed of a ceramic fiber and a binder, wherein the ceramic fiber contains substantially no shot and has an average fiber length of about 1 cm to about 10 cm, and the mat has an integral flexible structure and generates a substantially constant pressure over a temperature range of about 20 to about 1,200.degree. C., when the mat is fitted into the clearance. However, such a mat has a problem that it requires complicated drying processes, and further a ratio of the compression required upon mounting is considerably large, resulting in deterioration of working properties. In addition, since it is disclosed that from the consideration of the specific characteristic, i.e., the substantially "constant" pressure is generated when mounted into the clearance, it is suggested that the content of the organic binder resin in the mat is small, so that there is a tendency that the fiber in the mat is damaged by the frictional contact with the casing upon mounting. Accordingly, it is expected that deterioration in retaining force of the mat against the monolith, leakage of exhaust gases from the outer peripheral surface of the monolith are caused.
The present invention has been accomplished in order to solve the afore-mentioned problems. An object of the present invention is to provide a monolith-holding element and a process for producing the monolith-holding element, in which the monolith-holding element is preliminarily kept in a compressed state upon assembling of the catalytic converter, and therefore, has a small thickness so as to be readily mounted thereto; the monolith-holding element can exhibit a thickness-restoring property when heated and a required surface pressure between an outer peripheral surface of the monolith and an inner peripheral surface of a metal casing, and does not show any deterioration in properties after used for a long period of time; and the monolith-holding element is not corroded even when exposed to exhaust gases having a high-flow rate, and therefore, can maintain sufficient sealing properties.
Another object of the present invention is to provide a catalytic converter and a process for producing the catalytic converter, in which the afore-mentioned monolith-holding element is fitted into a clearance between the outer peripheral surface of the monolith and the inner peripheral surface of the metal casing; and the catalytic converter can withstand the high-temperature of exhaust gases and can stably support the monolith even when exposed to severe vibration and impact, so that there is no leakage of exhaust gases from the monolith-holding element.